Charge-switchable molecular magnet and spin blockade of tunneling

We analyze a model for a metal-organic complex with redox orbitals centered at both the constituent metal ions and ligands. We focus on the case where electrons added to the molecule go onto the ligands and the charge fluctuations on the metal ions remain small due to the relatively strong Coulomb repulsion. Importantly, if a nonzero spin is present on each metal ion it couples to the intramolecular transfer of the excess electrons between ligand orbitals. We find that around special electron fillings, addition of a single electron switches the total spin S-tot=0 to the maximal value supported by electrons added to the ligands, S-tot=3/2 or even S-tot=7/2 for metal ions with spin 1/2. This charge sensitivity of the molecular spin is a strong correlation effect due to the Nagaoka mechanism. Fingerprints of the maximal spin states, as either ground states or low-lying excitations, can be experimentally observed in transport spectroscopy as spin blockade at low bias voltage and negative differential conductance and complete current suppression at finite bias, respectively.